The subject matter disclosed herein relates to an air turbine starter and, more particularly, to an air turbine starter including a containment ring.
Aircraft engines, for example, gas turbines, are typically equipped with an air turbine starter (ATS) mounted on the engine accessory gearbox. The functional purpose of the ATS is to accelerate the engine up to a desired speed prior to ignition of the engine combustor and to continue assisting the engine start until the engine is capable of operating independently. The ATS is typically driven by pressurized air provided by an air source such as an auxiliary power unit, another operating engine, or an external air cart connected to the ATS. Pressurized air or gas fed into the ATS drives rotation of an ATS rotor causing rotation of a starter shaft. The starter shaft transmits this rotation to the drive shaft of the accessory gearbox. Rotation of the gearbox shaft drives rotation of a high pressure rotor of the engine which induces airflow into the engine and causes rotation of the engine high pressure rotor assembly. When the engine rotation reaches a desired speed, the ATS is turned off by stopping pressurized air flow.
A typical ATS has a turbine wheel with integral blades and a typical containment event is one where the entire turbine rim is shed in relatively large angular segments with the blades still attached. Containment events commonly occur because of some other system failure, which causes the turbine to become unloaded mechanically and go to free run speed. This generates excessive heat at the turbine bearings, which subsequently fail, and result in the turbine wheel translating axially. An area below the rim rubs against an adjacent stationary part, resulting in shedding of the rim. The blades themselves get rubbed down to some degree, transforming into smaller debris. These and/or other types of containment events must be considered when designing and building each ATS for aircraft engine and/or other uses.
To this end, one type of an ATS housing is known as softwall. A softwall is disposed within the ATS housing and includes multiple layers of a light weight penetration resistant fabric wrapped around a rigid but penetrable support ring. A containment event fragment will penetrate the support ring but be contained by the fabric. Softwall construction is expensive but light weight, a distinct advantage in an aircraft application. A second type of an ATS housing, known as hardwall, is disposed within the housing and includes a ring having sufficient radial thickness to resist penetration of a blade fragment. The choice of hardwall or softwall construction depends largely on the housing diameter. For a large diameter housing, hardwall construction is prohibitively heavy and therefore softwall construction, despite being expensive, is often employed. For a small diameter housing, the radial thickness required for penetration resistance imposes only a modest weight penalty and so the less expensive hardwall construction is usually employed.
Although hardwall construction is almost universally employed for small diameter housings, the thickness and rigidity of a hardwall often prevents ready deflection when struck by a fragment. Consequently, the full force of the impact is concentrated over a very short time interval and therefore may be quite damaging. The need to account for the abruptness and resultant severity of the impact contributes to the required thickness of the housing and therefore to its weight.